ABSTRACT
Cancer is one of the serious causes of debilitation and mortality in humans. Cancer develops as a function of age, influence of environment, diet, and genetic makeup, irrespective of man or animals. Carcinogenesis, also called tumorigenesis or oncogenesis, is the process of induction of tumors. Agents that induce these tumors are called carcinogens. Carcinogenesis is a multistep process by a series of mutations occurring at different levels. Cancer develops when the homeostasis between proto-oncogenes and tumor suppressor genes is disturbed and converted in oncogenes. This chapter explains the classification and theories of carcinogenesis by using easy and simple flowcharts to help the reader understand the process of carcinogenesis in head and neck cancers (HNCs).
